1. Field of the Invention
The present invention relates to a semiconductor device, more particularly, a semiconductor device that is suitable for use in connecting with electric condenser microphones.
2. Description of the Related Art
A condenser microphone (ECM) is an operational element for converting air vibrations, such as voice speech, into electrical signals in accordance with changes in capacitance between electrodes. Output signals from condenser microphones are extremely weak, so that it is necessary for an amplifying element, for amplifying the output signals from a condenser microphone, to have high input impedance, high gain and low noise.
Semiconductor devices suitable for such applications include junction type field-effect-transistors (J-FET) and metal-oxide-semiconductor (MOS) type FETs. J-FET has an advantage that the device can be readily produced in a bipolar-type integrated circuit (BIP-IC), as reported in a laid-open Japanese Patent Publication, 58-197885, for example.
FIG. 1 shows an example of the structure of the J-FET device (p-channel type). An n-type buried layer 3 is formed between layers 1 and 2. An island region 5 is formed by the p+-type separation regions 4 surrounding the buried layer 3 so as to connect the surface of the epitaxial layer 2 through to the subtrate 1.
An n+-type top gate region 6 is formed on the surface of the island region 5, and a p-type channel region 7 is formed in the lower layer of the top gate region 6. A p+-type source region 8 and a p+-type drain region 9 are formed, respectively, at each end of the channel region 7, and gate contact regions 10 of a high n-dopant concentration are formed on each respective outer region.
Finally, a p-channel type J-FET is produced by fabricating a source electrode 11S, a drain electrode 11D and a gate electrode 11G with an intervening insulation layer 16. Utilizing the p-n junction formed in the gate region, and reverse-biasing this region, controls the strength of the depletion layer thereby controlling the drain current.
When such a circuit configuration is integrated, a p-type base region 12, an n+ type emitter region 13 and an n+ type collector region 14 are formed in the other island region 5, so that an integrated circuit network comprised by the n-p-n transistors and so on, amplifies signals received in the J-FET device.
However, to use such a device for amplifying signals from an electric condenser microphone, it is necessary, in some cases, to provide an expansion electrode 15 of an area much larger than the area of electrode pads (bonding pads) on the integrated circuit.
When such a structure is fabricated, parasitic capacitances are produced between a capacitor C1 formed by the expansion electrode 15 and the epitaxial layer 2 with the intervening insulation layer 16, on the one hand, and a p-n junction capacitor C2 formed by the epitaxial layer 2 and the substrate 1, where both capacitances become grounded to the substrate 1 biased at the ground potential GND. The magnitude of such parasitic capacitances can reach several tens of pF so that the detrimental effects can reach a level that is not to be ignored.
FIG. 2 shows a schematic circuit diagram that includes the capacitances C1, C2. One end of the ECM is connected to the gate electrode (input terminal) of J-FET 17, and the source electrode of the J-FET 17 is grounded, and the drain electrode is connected to the output terminal OUT. The output terminal OUT is connected to the integrated circuit comprised by n-p-n transistors. The capacitances C1, C2 are connected in series between the gate electrode of the J-FET 17 and the ground potential GND. In such a circuit, output signals from the ECM flows from the ECM to the ground potential GND (shown by current i in the diagram), resulting a problem that the signal level to be impressed on the gate terminal of J-FET 17 is reduced so that desirable output voltage from the ECM cannot be obtained.